I'm considering this to be a geek investment. I enjoy overclocking and building high performance computers, so I figured why not. If the market ever were to crash, I'm always able to resell the graphic cars on eBay etc. Though if I invested money in buying in actual BTC, then I'd be screwed. (and they would be worthless)
I feel the same in enjoying building rigs and doing it for a geek investment, 
but there is one thing that still worries me. If the BTC market crashes, *everybody* will be selling their equipment. And the price for all those low-end cards will plummet following the value of BTC. 
My one rule is to NEVER buy BTC - only buy hardware to mine BTC.
I'm not so sure about this. Why not buy if the value goes down? But then you are like any Wall Street trader, I guess. 